1. Technical Field
The present invention relates to the field of metrology, and more particularly, to target designs for measuring unresolved device-like pitches.
2. Discussion of Related Art
On-device overlay (OVL) metrology is challenging as the design rule pitches are unresolved by modern optical tools (both imaging and SCOL—scatterometry overlay tools). Instead, OVL measurement is performed on specially designed “proxy” targets, having typical scales (pitches) of hundreds of nanometers and the gap between device pitch (<90 nm) and the “proxy” target pitch is constantly growing. Since all processing steps are optimized to device scales the “proxy” targets are not fully process compatible which results in various types of target asymmetries appearing in OVL targets.
Using the Moiré principle enables resolving small pitches, as the resolved pitch resulting from mutual re-scattering of diffraction orders between gratings with small unresolvable but close pitches is
      P    =                            p          1                ·                  p          2                                      p          1                -                  p          2                      ,so that choosing
            p      1        -          p      2        ⪡                    p        1            +              p        2              2  one can get
  P  ⪢                    p        1            +              p        2              2  as the resolved pitch which relates to the unresolved pitches p1, p2.
U.S. Pat. No. 7,440,105 as well as Ausschnitt 2001 (“From compliance to control: off-roadmap metrology for low-kJ. Lithography”, Metrology, Inspection, and Process Control for Microlithography XV, Neal T. Sullivan, Editor, Proceedings of SPIE Vol. 4344), which are incorporated herein by reference in their entirety, suggest using Moiré patterns from one-dimensional gratings (grating over grating side by side), however, low re-scattering efficiency and gratings removed from each other at more than the decay distance (diffraction orders corresponding to device-like pitches are strongly evanescent with decay length≦100 nm) limit the applications.